The invention relates to a method of providing a metal pattern on a glass substrate in an electroless process, in which method the substrate is provided with a silane layer which is subsequently exposed to actinic radiation in accordance with a pattern, after which the substrate is brought into contact with an aqueous metal-salt solution, thereby forming the metal pattern on the unexposed areas of the substrate.
The invention relates in particular to a method of manufacturing a black matrix of metal on a passive plate of a liquid crystal display device.
Electroless or chemical metallization is a simple and inexpensive method of metallizing dielectric substrates such as glass and synthetic resins. For this purpose electroless metallization baths, such as copper and nickel baths, are used which comprise complexed metal ions and a reducing agent. On catalytic surfaces the metal ions are reduced to metal. In general, metallic Pd nuclei are provided on the surface to be metallized in order to render the surface catalytic. In a standard procedure the substrate to be metallized is nucleated beforehand (termed activation) by bringing the substrate into contact with either aqueous solutions of, in succession, SnCl.sub.2 and PdCl.sub.2 or with a colloidal SnPd dispersion. As a result thereof Pd nuclei are formed on the surface to be metallized. Subsequently, the activated surface is immersed in an electroless metallization bath, causing the surface to be metallized. Such activation methods are non-selective, i.e. the entire substrate surface, such as glass, is nucleated and hence metallized. These activation methods, in which the strongly reducing formaldehyde is used as the reducing agent, can suitably be used for electroless copper. However, for most electroless nickel baths such activation methods are less suitable due to the reduced reactivity of the reducing agents, for example hypophosphite, used in these baths. This is caused by adsorbed Sn.sup.4 +particles which are used as stabilizers in electroless nickel baths, but which also inhibit the oxidation of the reducing agent.
In electronic applications, selective or patterned metallization is often desired. This can be attained in various ways. In a subtractive process, first a uniform metal layer having the desired thickness is deposited on the substrate. Subsequently, a photoresist layer is provided which is exposed in accordance with a pattern and developed, thereby forming a pattern in the resist layer. Finally, the metal layer is etched selectively after which the resist layer is stripped off. In an additive process the substrate is activated with catalytic Pd nuclei. Subsequently, a photoresist layer is provided on the substrate, exposed in accordance with a pattern and developed, thereby forming a pattern in the resist layer. Subsequently, the surface is immersed in an electroless metallization bath, in which process metal is deposited in the desired thickness in the aperture of the resist pattern. Finally, the resist layer is stripped off and the Pd nuclei are removed by a short etching treatment. Both processes have the disadvantage that they require a relatively large number of process steps and involve the use of chemicals which are harmful to the environment, such as the resist stripper and the metal-etching bath. In addition, the provision of resist layers on large glass surfaces is rather difficult.
It is also known to apply a Pd-acetate film to a glass plate by means of spin coating, which film is locally decomposed to metallic palladium by means of a laser. The Pd acetate on the unexposed parts is subsequently removed. The pattern of Pd nuclei thus formed is then metallized in an electroless nickel or copper bath. The disadvantage of this method is the relatively large number of process steps and the high laser power necessary to decompose the Pd acetate. Consequently, the treatment of large glass surfaces is very time-consuming.
In U.S. Pat. No. 4,996,075 a description is given of a method of depositing a very thin silver film in accordance with a pattern on a SiO.sub.2 surface. In this method the surface is treated with a solution of a silane with a vinyl or acetylene group in an organic solvent such as carbon tetrachloride and chloroform. In this treatment a monomolecular silane layer is formed on the SiO.sub.2 surface, i.e. a silane layer having a thickness equal to the length of the silane molecule is formed. Local irradiation of the silane layer with an electron beam causes the vinyl or acetylene groups to be chemically bonded to one another, thereby forming a polymer layer, and hence to be selectively deactivated. Subsequently, the surface is first immersed in a solution of diborane in THF and then in an alkaline solution of hydrogen peroxide, so that the unexposed vinyl groups are converted to hydroxyl groups. Subsequently the hydroxyl groups are converted to aldehyde groups. A treatment with an aqueous silver nitrate solution causes the silver ions to be reduced by the aldehyde groups to metallic silver, thereby forming a patterned silver layer having a thickness of one atom layer in the unexposed areas. A second monomolecular layer of vinyl silane can be formed on the silver oxide layer obtained by spontaneous conversion of the monoatomic silver layer to a monomolecular silver oxide layer, after which the above steps for converting vinyl groups via hydroxyl groups into aldehyde groups are repeated. Subsequently a second treatment with an aqueous silver nitrate solution is carried out, which results in the formation of a second monomolecular silver oxide layer. By repeating these steps many times an alternating laminate of monolayers of silane and monolayers of silver oxide is obtained.
A disadvantage of this known method is the large number of process steps required to obtain a metal pattern having of sufficient layer thickness, for example 0.1 .mu.m or more, so that the layer is optically tight and/or has a sufficiently low electric resistance. Another disadvantage is the use of harmful organic solvents as a solvent for the silanes with a vinyl or acetylene group. A further disadvantage is formed by the fact that the proposed irradiation of the silane layer causes said layer to be deactivated by mutual bonding of the vinyl or acetylene groups, thereby forming a polymer layer which covers the SiO.sub.2 surface. This polymer top layer cannot easily be removed and is often undesired. Due to said polymer layer the SiO.sub.2 surface is inaccessible to other surface reactions or causes, for example, bonding problems with other layers to be provided.